diff --git a/plugins/outputs/graylog/README.md b/plugins/outputs/graylog/README.md
index d59602148..2b0475263 100644
--- a/plugins/outputs/graylog/README.md
+++ b/plugins/outputs/graylog/README.md
@@ -4,6 +4,23 @@ This plugin writes to a Graylog instance using the "[GELF][]" format.
 
 [GELF]: https://docs.graylog.org/en/3.1/pages/gelf.html#gelf-payload-specification
 
+## GELF Fields
+
+The [GELF spec][] spec defines a number of specific fields in a GELF payload.
+These fields may have specific requirements set by the spec and users of the
+Graylog plugin need to follow these requirements or metrics may be rejected
+due to invalid data.
+
+For example, the timestamp field defined in the GELF spec, is required to be
+a UNIX timestamp. This output plugin will not modify or check the timestamp
+field if one is present and send it as-is to Graylog. If the field is absent
+then Telegraf will set the timestamp to the current time.
+
+Any field not defined by the spec will have an underscore (e.g. `_`) prefixed
+to the field name.
+
+[GELF spec]: https://docs.graylog.org/docs/gelf#gelf-payload-specification
+
 ## Configuration
 
 ```toml